1. Field of the Invention
The present invention relates to electrostatic ink-jet printing heads, and more particularly, for ejecting toner particles from ink ejecting ports onto a printing medium under influence of electrostatic power.
2. Description of the Related Art
Recent attention has been focused on a non-impact printing method in which printing noise generated during printing is reduced to a negligible level. Among such methods, an ink-jet printing method has an advantage to enable high-speed printing directly on a printing medium, and also even on plain paper, with use of a simple mechanism.
Various sorts of ink-jet printing methods have been proposed so far. One of such ink-jet methods is an electrostatic ink-jet printing method. In this method, a plurality of ejecting electrodes are provided in front of printing paper and a single counter-electrode is provided in back thereof. When a voltage is applied across the plurality of ejecting electrodes and counter-electrode, an electrostatic force of electric field is generated between them and colored material such as ink is ejected toward the printing paper for printing. The ink used in this method contains charged toner particles in a carrier liquid as an insulating solvent.
An example of an electrostatic ink-jet printing head is disclosed in Japanese Patent Application Laid-open Publication No.60-228162. FIG. 19 is a perspective view of this prior art printing head. The printing head 1 comprises a head base 2 made of insulating material and an upper cover 3 provided opposed thereto. On a surface of the head base, a plurality of ejecting electrodes 4 are provided. The head base has a plurality of protrusions 5 underlying the respective ejecting electrodes 4. The upper cover 3 is positioned on the head base 2 recessed from the ends of the protrusions 5. Between the head base 2 and the upper cover 3, slit-like ink ejecting opening 6 is defined. The ink ejecting opening 6 is connected to ink tank portion 7 which the head base 2 and upper cover 3 also define therebetween. The ink stored in the ink tank portion 7 is supplied to the ink ejecting opening 6 by capillarity. The ink positioned at the ink ejecting electrode 4 has a pointed shape according to corresponding protrusion 5. Because of the shape, the electrical field caused by applying high voltage on an ejecting electrode 4 focuses the ink in the vicinity of ejecting electrode 4 and the ink is easily propelled to paper.
Another example of an electrostatic ink-jet printing head is disclosed in Japanese Patent Application Laid-Open Publication Nos. 58-153661 and 58-153662. FIG. 20 is a perspective view of major parts of this prior art printing head, and FIG. 21 is a cross-sectional view thereof. In the ink-jet printing head disclosed in these two publications, a plurality of nozzles 12 having a first electrode 13, 14 on an inner wall of each nozzle or in the vicinity thereof are disposed in a two-dimensional X and Y array within a head base 2, and the first electrode 13, 14 forms a common lead terminal with respect to a Y direction (printing-paper feeding direction) of the nozzle group. A second electrode 15, 16, 17 common with respect to the X direction (printing-paper width direction) of the nozzle group is disposed behind the first electrode, and a third electrode (not shown) is disposed in front of the first electrode, i.e., on its printing side, so that a generated electric field causes toner particles in ink 18 to be ejected from the nozzle group onto a printing paper to thereby print an image on the paper.
In this latter prior art, however, every nozzle respectively has a electrode on an inner wall of the nozzle or in the vicinity thereof disposed in a two-dimensional X and Y array, and the first electrode forms a common lead terminal with respect to a Y direction (printing-paper feeding direction) of the nozzle group or in an X direction (printing-paper width direction). The structure of these nozzles is so complicated that this structure has made it difficult to reduce printing-head manufacturing costs.
Further, the slit-like ink ejecting opening of the former prior art is formed as very narrow slit and the nozzles of the latter prior art are formed as very small holes to suppress an ink flow in the vicinity of the end of the opening or nozzles to be low to thereby prevent ink leakage. For this reason, in openings or nozzles whose ink ejection frequency is low, excessively concentrated toner particles cause the toner particle density to become high to thereby increase the ink viscosity. This causes generation of improper ejecting of the ink droplet, which leads to a reduction in the printing quality of the printing paper.